1. Field of the Invention
The present invention relates to a process for producing single crystal diamond film which is suitable for use in electronic devices (such as transistors, diodes, and sensors), heat-sinks, surface acoustic wave devices, X-ray windows, optical materials, antiwear materials, decorative materials, and other coating materials.
2. Description of the Prior Art
Diamond is characterized by its outstanding heat resistance and large band gap (5.5 eV). It is usually an insulator, but can be semiconducting by doping with impurities. Diamond also has good electrical properties, such as a high breakdown voltage, a high saturated drift velocity, and a low dielectric constant. Because of these electrical characteristics diamond is considered to be an excellent material for use in electronic devices and sensors to be used at high temperatures., high frequencies, and high electric fields.
So far, studies have been conducted on the use of diamond in photosensors and light-emitting elements for light in the short-wave length region (such as ultraviolet) based on the large band gap, in heat sinks based on the high thermal conductivity and low specific heat, in surface acoustic wave devices based on the extremely high hardness, and in X-ray windows and optical materials based on the high transmittance and refractive index. Furthermore, diamond has been widely used as an antiwear material for cutting tools.
In order to fully exploit the characteristics of diamond in various fields, it is necessary to synthesize high quality single crystal diamond with low structural defects. Moreover, for practical use, it is necessary to develop processes for depositing single crystal diamond films on large areas at low cost. At present, single crystal diamond is obtained by natural mining or synthesis under high pressure--high temperature conditions. However, such single crystal diamond, which will be referred to as bulk diamond, has only a limited crystal surface area of about 1 cm.sup.2 at the largest. Moreover, it is extremely expensive. Therefore, its industrial use is limited only to specific areas, such as abrasives and high precision cutting tools.
It is well known that polycrystalline diamond films can be synthesized from gas phase by microwave chemical vapor deposition (CVD) (Japanese Laid-Open Patent Application Nos. 27754/1984 and 3320/1986), high-frequency plasma CVD, hot filament CVD, DC plasma CVD, plasma jet method, combustion, or hot filament CVD. These methods permit the economical production of diamond film of a large area.
However, diamond films grown on non-diamond substrates such as silicon are polycrystalline where diamond particles randomly aggregate, and as shown in the electron micrograph of FIG. 5, a large number of grain boundaries are contained in the film. Recently, it has been reported that highly oriented diamond films, where diamond crystal grains are oriented in almost the same orientation, as shown in FIG. 6, can be synthesized. However, the film is also polycrystalline, and still contains a large number of grain boundaries. Since grain boundaries trap and scatter carriers (electrons and holes) moving through diamond, even the highly oriented diamond film is inferior in electrical properties to bulk diamond with few grain boundaries. Consequently polycrystalline and highly oriented diamond films do not exhibit satisfactory performance for practical electronic devices and sensors.
Likewise, the existence of grain boundaries is disadvantageous for optical applications because of light scattering and low transmittance. In applications for cutting tools, polycrystalline diamond films are liable to chip.
When single crystal bulk diamond or cubic boron nitride is used as the substrate in the gas-phase synthesis, single crystal diamond films can be grown. However, its surface area is limited as mentioned hereinabove.
It is known that diamond films with a certain degree of grain orientation can be synthesized if nickel and copper are used as the substrate. However, nickel becomes brittle in the high temperature hydrogen plasma atmosphere for the gas-phase synthesis of diamond, and reacts with diamond formed thereon, thereby converting diamond into graphite (D. N. Belton and S. J. Schmeig, J. Appl. Phys., Vol. 66, p.4223 (1989)). On the other hand, copper has a linear thermal expansion coefficient which is more than 10 times greater than that of diamond at temperatures above 600.degree. C., so that the diamond film formed on copper substrate is liable to be peeled off after cooling down to room temperature (D. F. Denatale et al., J. Materials Science, Vol. 27, p. 553 (1992)).
Gas-phase synthesis of diamond on platinum and other transition metals have been studied. However, only polycrystalline diamond films were grown (Matsumoto and Takamatsu, "Hyomen Gijutsu", Vol. 44, No. 10, p. 47 (1993); M. Kawarada et al., Diamond and Related Materials, Vol. 2, p. 1083 (1993); D. N. Belton and S. J. Schmeig, J. Appl. Phys., Vol. 69, No. 5, p.3032 (1991); D. N. Belton and S. J. Schmeig, Surface Science, Vol. 223, p. 131 (1990); and Y. G. Ralchenko et al., Diamond and Related Materials, Vol. 2, p. 904 (1993)).
For industrial use of diamond films, it is necessary to develop a process for synthesizing single crystal diamond films, which is completely or almost completely free of grain boundaries, on a large area. Such a process has not been available so far.